1. Technical Field
This disclosure relates generally to a flow battery system and, more particularly, to a system and method for operating a flow battery system at an elevated temperature.
2. Background Information
A typical flow battery system includes a stack of flow battery cells, each cell having an ion-exchange membrane disposed between negative and positive electrodes. During operation, a catholyte solution flows through the positive electrode, and an anolyte solution flows through the negative electrode. The catholyte and anolyte solutions each electrochemically react in a reversible reduction-oxidation (“redox”) reaction. Ionic species are transported across the ion-exchange membrane during the reactions, and electrons are transported through an external circuit such as a power converter to complete the electrochemical reactions.
An example of a pair of catholyte and anolyte solutions is a pair of vanadium/vanadium solutions. The vanadium catholyte solution typically includes a plurality of V4+ and/or V5+ ions. The vanadium anolyte solution typically includes a plurality of V2+ and/or V3+ ions. Ideally, the concentrations of these vanadium ion species should be as high as possible in order to minimize the size of the tank required for a given amount of energy storage; i.e., higher concentrations enable a flow battery system with a higher energy density. However, the concentrations are limited by the solubility of the vanadium salts in the solvent electrolyte, which is typically an aqueous acid such as sulfuric acid. Additionally, the solubility of these different vanadium salts (e.g., vanadium sulfates) vary with the temperature of the solution. The V2+, V3+ and V4+ salts are generally less acid soluble at lower temperatures. The V5+ ions, on the other hand, are generally less acid soluble at higher temperatures. An additional complication is that the concentrations of the different oxidation states may vary with the state-of-charge (SOC) of the battery and, ideally, one would like the salts to remain in solution over a wide range of SOC (e.g., from 0 to 100% SOC, such that salt solubility does not limit the minimum or maximum SOC). For example, a typical electrolyte composition used in a vanadium redox battery system is an aqueous solution of approximately 1.5 to 2.0 molar (M) vanadium sulfate and 1.5 to 2.0 M sulfuric acid for both the anolyte and the catholyte. The anolyte and catholyte composition enables an operating range of approximately zero to forty degrees Celsius, with the lower temperature limit determined by the solubility of the V2+, V3+ and V4+ salts and the upper temperature limit determined by the solubility of the V5+ salt. Vanadium flow battery systems, therefore, are typically operated within a relatively narrow temperature range (e.g., approximately zero and forty degrees Celsius) to prevent formation of metal salt precipitates. A wider temperature window would be beneficial since a lower minimum temperature would eliminate the need for “freeze” prevention measures and a higher maximum temperature can enable improved cell performance, as well as improved heat rejection to the environment (especially on hot days where ambient temperatures are close to, or may even exceed, forty degrees Celsius).